Controls of basin margin tectonics on the morphology of alluvial fans in the western Ganga foreland basin's piedmont zone, India

Abstract

Six distinct alluvial fans have developed in the western Ganga basin's piedmont zone between the Ganga (aka Ganges) River in the west and the Sarda River in the east. The basin margin is demarked by the active Himalayan Frontal Thrust along which the Siwalik ranges are uplifted and placed over the alluvia of the Ganga basin. The 6 alluvial fans show marked morphological variances, but their morphometric parameters exhibit proportionality relationships with the “mountain‐front sinuosity index (S mf ),” “stream‐gradient index (SL),” and “valley‐floor width to height ratio (V f )” of the adjoining mountain‐front segments. Correlation and regression analyses reveal that these relationships are statistically significant. The size of the fan is directly proportional to the magnitude of S mf and inversely proportional to the magnitude of SL, whereas the fan gradient is inversely proportional to S mf . The higher the S mf and the lower the SL values, the larger and gentler is the alluvial fan. Given that the studied alluvial fans have developed in the same climatic zone and their drainage basins have similar physiographic, geologic, and climatic conditions, the morphological diversities among the alluvial fans are attributed to the tectonics of the basin margin. Moreover, statistical analyses further rule out any significant relationships of the fan size with area, or drainage density of the drainage basin. The lower values of S mf and V f and higher values of SL indicate a more active nature and, thus, greater/faster uplift of the mountain‐front and expected deepening of the accommodation space in the adjoining basin, which favoured vertical aggradation and development of smaller and steeper alluvial fans. Conversely, the higher values of S mf and V f and lower values of SL indicate a less active nature and, thus, relatively lower/slower uplift of the mountain‐front and thus creation of shallower accommodation space in the adjoining basin, favouring the development of larger and gentler alluvial fans.